1. Field of the Invention
The present invention relates to an optical apparatus which incorporates focusing and magnification changing lenses and lens barrel structural mechanisms such as a lens holding mechanism and a lens shifting mechanism.
2. Description of the Related Art
In recent years, rapid progress has been made in the field of optical apparatuses such as cameras in regard to miniaturization of photographing or imaging optical systems and of image sizes of photographing films and solid-state imaging devices. It is also a current trend that plastics are commonly used as the materials of the photographing optical systems and members for holding such optical systems.
The use of plastic materials as the materials of optical systems and holding members offers a great advantage as compared with the case where other kinds of materials are used. For instance, optical elements and members for holding such elements can easily be fabricated by molding, with a large degree of freedom of sizes and shapes. In addition, use of plastic materials is advantageous also from the viewpoint of costs of production. For these reasons, plastics are finding spreading use as the materials of lenses and optical system holding members in finder systems, infrared active focusing units and photographing optical systems.
In general, plastics exhibit greater amounts of changes of physical properties in response to changes in environmental conditions, as compared with inorganic glass materials. For instance, while an organic glass LaK 14 (produced by OHARA) exhibits a coefficient of linear thermal expansion of 57.times.10.sup.-7 /.degree.C., a plastic material PMMA exhibits a coefficient value of 57.times.10.sup.-6 /.degree.C. which is one order of magnitude greater than that of the LaK 14. As to the change in the refractive index, PMMA exhibits a value of 1.0.times.10.sup.-4 /.degree.C. to 1.2.times.10.sup.-4 /.degree.C., which is two orders of magnitude greater than 3.9.times.10.sup.-6 /.degree.C. to 4.4.times.10.sup.-2 /.degree.C. exhibited by the LaK 14 for D rays.
Thus, the plastics exhibit much greater changes of physical constants (refractive index and shapes) in response to change in temperature, as compared with inorganic glass materials. For instance, lenses made of plastics, i.e., so-called plastic lenses, show a considerably greater change of focal length in response to change in temperature, as compared with lenses made of glasses.
Furthermore, the plastics shows greater water absorption than inorganic glass materials. Consequently, optical constants of plastic lenses show a greater amount of change than those exhibited by inorganic lenses in response to change in humidity, as well as to change in temperature.
Thus, the use of plastics as the material of parts of optical systems and members for holding the optical systems poses a problem of greater degrees of change in physical properties and dimensions in response to change in environmental conditions, in particular change in temperature and humidity, although the plastics offer advantages as stated before. For instance, the use of plastics causes various inconveniences in regard to optical performance such as large change in the optical properties such as focal length which is much less critical when inorganic glasses are used as the materials.
Thus, problems have hitherto been pointed out that focal position of an optical system is undesirably varied to create a de-focus condition due to change in environmental conditions, so as to hamper adequate lens control. Influence of de-focus of optical system due to change in environmental condition is becoming more serious because of current trend towards higher image quality realized by digital techniques.
Manufacturers are in a severe competition towards greater magnification of zoom lenses and miniaturization of the same. The de-focus of the optical system due to change in temperature and humidity is quite a significant problem which may cause a critical defect of the products.
Under these circumstance, it is very important that an effective compensation is made for deviation of focal plane caused by changes in environmental conditions.
This requirement will become much more important under the scheme towards higher image quality, greater magnification and miniaturization.
Compensation for the effect produced by changes in environmental conditions have to employ different compensation coefficients when different amounts of compensation have to be employed for different pieces of products or different production lots of products. From the viewpoints of schedule of development, number of steps of production process and production costs, it is quite inconvenient to change a main program each time a different compensation coefficient is adopted.
Problems which are quite serious in optical apparatuses are also caused such as de-focus caused when the optical apparatus is used under conditions different from the conditions under which the optical apparatuses were initially or eventually adjusted.